The present invention relates to Automatic Gain Control (AGC) in a radio receiver, and more particularly technology that optimizes AGC in scenarios in which only a portion of a full transmission bandwidth is received at a time.
One of the most important properties of a hand held device is its battery life. For a User Equipment (UE) in a mobile communication system, the dominating power consumer is the radio. In some cases, a UE's radio circuitry can be responsible for more than 50% of the total power consumption. Thus, minimizing the amount of time the radio needs to be active is crucial for extending the UE battery life.
Due to the limited dynamic range of the receiver, the UE needs to adjust the gain prior to reception. An AGC algorithm, responsible for this adjustment, typically starts by setting one or more amplifiers to operate at some initial gain value. The power of the received signal is then measured, and the gain is adjusted accordingly. This procedure is then repeated until a good gain value has been found. If a good initial guess is available, the algorithm typically requires less radio time to converge. Because the gain has to be corrected prior to reception of useful data, one must know how long the AGC algorithm needs to achieve convergence so that reception can be started at the correct time. Thus, to be able to reduce the additional time needed prior to reception, one must know a priori that an optimized algorithm can be used; otherwise, the worst-case time to convergence must be assumed. In some situations it might even be possible to use a gain value from a previous run of the AGC algorithm.
AGC algorithms generally consist of three steps: power measurement, gain computation, and actuation. The purpose of the power measurement step is to estimate the received power of the signal. In the gain computation step, a suitable gain value is determined; and in the actuation step, this gain value is applied to the receiver chain.
To be able to make a reliable estimate, the signal on which the estimate is based needs to be representative of the signal one wants to receive. For example in the case of Evolved Universal Terrestrial Radio Access (E-UTRA), some subframes might be allocated to Multimedia Broadcast over Single Frequency Network (MBSFN) transmissions, or in the case of Time Division Duplex (TDD) mode, might be allocated for uplink transmission. In each of these examples, these subframes are not representative of the wanted signal and thus are not suitable for power measurements. For example, MBSFN subframes might be used by the Evolved NodeB (eNB) for power saving purposes and thus contain no or at least very little power compared to subframes allocated for regular downlink transmissions. Even if all cells are not synchronized in time, the received power might be dominated by a nearby eNB. Thus the received power during MBSFN subframes might be very low compared to regular subframes. It will further be recognized that subframes allocated for uplink transmission cannot be used to estimate the power of downlink subframes. Thus, the UE must limit power measurements to certain suitable time intervals. This is has the unwanted effect that the AGC procedure can be quite time consuming.
FIGS. 1 and 2 illustrate the limited time available for making suitable power measurements for the cases of Frequency Division Duplex (FDD) and TDD operation, respectively. In the figures, in addition to abbreviations already introduced, the following abbreviations are used:
P-SCH: Primary Synchronization Channel
S-SCH: Secondary Synchronization Channel
TX: transmission
GP: Guard Period
UL: uplink
More particularly, FIG. 1 depicts synchronization signals and reference symbols transmitted in an FDD cell. Only the central 72 sub-carriers are shown. Some sub-frames may be used for MBSFN, to take one example, and hence might not contain cell-specific reference symbols other than in the first symbol.
FIG. 2 depicts synchronization signals and reference symbols transmitted in a TDD cell. Only the central 72 sub-carriers are shown. Some sub-frames may be used for UL transmissions and hence might not contain cell-specific reference symbols, while others may be used for downlink (DL) transmissions but used for MBSFN transmissions, and hence contain reference signals only in the first symbol.
During the actuation phase of the typical AGC algorithm, the gain value is typically changed in multiple places through the receiver chain. These changes impair the received signal with, for example DC-transients and phase discontinuities. Thus gain changes should be limited to moments in time at which the impact of these impairments will be limited (i.e., they will not degrade reception of data). In the case of E-UTRA channel reception, these changes can for example be limited to occur at slot or subframe borders.
A measure of the frequency channel variations is the so called coherence bandwidth, Bc≈1/τmax where τmax is the maximum delay spread (difference between first and last significant tap in the impulse response). The coherence bandwidth for the three typical channel profiles (i.e., Extended Pedestrian A (EPA), Extended Vehicular A (EVA) and Extended Typical Urban (ETU)) used in 3GPP are 2.44 MHz, 0.40 MHz and 0.2 MHz. Exemplary frequency responses for these three channels are illustrated in respective FIGS. 3a, 3b, and 3c for the case of a 20 MHz cell in good coverage conditions.
As a comparison, FIG. 4 illustrates the frequency response for a 3GPP ETU channel for a 20 MHz cell in low coverage conditions (−10 dB SNR).
Conventional receiver equipment applies a “better safe than sorry” approach with respect to AGC. That is, the AGC is always scheduled to run, regardless of whether it is really needed. This conservative approach results in unnecessarily high power consumption under some circumstances.